1. Field of the Invention
This invention relates to a semiconductor substrate and a process for producing the same, and particularly to a process for producing a semiconductor substrate comprising a light-transmitting insulator substrate having thereon a single-crystal semiconductor layer. More particularly, this invention is concerned with a semiconductor substrate comprising a light-transmitting insulator substrate made of glass or the like on which a single-crystal semiconductor for fabricating a device is formed thereon afterwards by epitaxy or ion implantation.
2. Related Background Art
Formation of single-crystal silicon semiconductor layers on insulating materials are widely known as silicon-on-insulator (SOI) techniques, and much research has been conducted on SOI, since substrates thereby obtained have a number of advantages that can not be achieved by bulk silicon substrates for preparing silicon integrated circuits.
If single-crystal films grown on insulating films can be used as substrate materials for devices, it is favorable for the device structure. Devices with a high performance (a high speed) and a high reliability can be achieved because 1) any parasitic (or floating) capacitance caused by substrate components can be reduced, 2) devices can be resistant to radiation and 3) latchup-free CMOS can be achieved. Hence, SOI devices have attracted attention.
Among SOI formation techniques recently reported, a technique especially superior in view of quality is what is called "bonding SOI". This is a technique in which mirror-finished surfaces of two wafers on at least one of which an insulating film has been formed by oxidation or the like are closely brought into face-to-face contact, followed by heating to strengthen the bond at the interface of close contact, and then the substrate is polished or etched on either side thereof so as to leave on the insulating film a single-crystal silicon thin film having any desired thickness. What is most important in this technique is the step of making the silicon layer into a thin film. This is because the above advantages of SOI can not be brought out unless silicon layers are made into thin films.
However, in order to make silicon layers into thin films, usually a silicon substrate of as large as several hundred .mu.m must be uniformly polished or etched to have a thickness of several .mu.m or 1 .mu.m or less. It is technically very difficult to control such polishing or etching or to make the substrate thickness uniform. Because of such a difficulty in film thickness control, this "bonding SOI" has not been put into practical use irrespective of its possibility of providing the best-quality single-crystal thin films among SOI techniques.
The bonding SOI technique has another important problem, the difference in coefficient of thermal expansion between insulator substrates and silicon substrates. This difference in coefficient of thermal expansion comes into question to a small degree when the silicon substrate is used on the side of the substrate serving as a support (i.e., when silicon substrates are bonded to each other). When, however, substrates having coefficients of thermal expansion which greatly differ from each other are bonded and any temperature changes occur, the difference in coefficient of thermal expansion between both substrates causes a stress.
In practice, when an insulator substrate made of glass, other than silicon, is used on the side of the substrate serving as a support, at the step of heating them at about 1,000.degree. C. in order to strengthen the bond at their interface the substrates may warp or the substrates may break or separate, because of such a difference in coefficient of thermal expansion between both substrates. There are examples in which materials having a coefficient of thermal expansion close to that of silicon are synthesized and used as the supporting substrate. Such materials, however, have a poor thermal resistance as far as is known in the art, and can not withstand process temperatures for the heat treatment applied to strengthen the bond or for the fabrication of devices.
Abe et al. report an example in which a "bonded SOI substrates" that can solve such problems is produced (extended Abstract of the 1992 International conference on SOLID-STATE DEVICES AND MATERIALS, 1992 Tsukuba, pp. 437-439, or Japanese Patent Application Laid-open No. 4-286310).
In the process reported therein, a relatively thin (300 .mu.m) silicon substrate and a quartz substrate are bonded, and thereafter a first heating is carried out at about 300.degree. C. that may cause no separation break of the bonded substrate, followed by etching only the silicon substrate to make it thinner to have a thickness of about 150 .mu.m. Next, as a second heating, annealing is carried out at about 450.degree. C., to attain a bond strength high enough to withstand the shear stress during surface grinding, and the silicon substrate is then ground down to a thickness of several .mu.m by means of a grinder. Thereafter, precision polishing is further carried out to make the silicon substrate into a thin film.
According to this process, however, since the heating is essential, a thin silicon substrate about 300 .mu.m thick must be used taking account of thermal stress. Hence, substrates may accidentally break during operations for bonding or transportation of substrates, and hence the operations must be carefully made. Moreover, in order to carry out heating at higher temperatures, it is necessary to repeat a cycle of the grinding to make substrates thinner and the heating is carried out thereafter. For these reasons, bonding SOI substrates are disadvantageous because their production speed can not be increased.
Stated specifically, the substrate thickness of the semiconductor substrate to be bonded must usually be about 500 .mu.m in the case of a silicon substrate of 4 inches in diameter and about 600 .mu.m in the case of that of 5 to 6 inches in diameter so that their mechanical strength can be maintained. In the case of a larger substrate of 8 inches in diameter, a silicon substrate about 800 .mu.m thick must be used. When a thin substrate about 300 .mu.m thick is used, it becomes very difficult to handle the step of initial bonding.
There is still another disadvantage. It is a problem of separation caused by the shear stress applied between the insulator substrate and the semiconductor substrate. Every time silicon substrates are ground down to be made thinner, a great shear stress is applied to the bonding interface between the supporting substrate and the silicon substrate. In practice, silicon substrates are ground down or polished until they are made into thin films of several .mu.m thick and hence a quite large shear stress is applied to the bonding interface. Moreover, at the joining surfaces of the substrates, the bond at the bonding interface may become weaker with the repetition of grinding. To solve this problem, there is a method in which the step of making substrates thinner by grinding and the high-temperature heating are repeated so that the bond strength at the interface does not deteriorate. This method, however, takes a long time for processing and is not suited for mass production.
Still another disadvantage is that, since single-crystal silicon thin films are produced by polishing, a special apparatus and a very precise control are required to achieve uniformity of film thickness.
As another process for producing SOI substrates, there was a means of direct deposition of semiconductor films on insulator substrates. However, the semiconductor film can not well crystallize on the insulator substrate, and it is impossible to form single-crystal semiconductor thin films.
As discussed above, conventional "bonding SOI substrates" cannot achieve sufficient bond strength between the insulator substrate and the silicon substrate without heat treatment. On the other hand, as previously noted, the direct bonding of a silicon substrate and a transparent substrate having different coefficients of thermal expansion and the subsequent heating of the bonding cause the break or warpage of the substrates. To solve such problems, up to now the heating has been delicately controlled under conditions such that a bond strength high enough to withstand the shear stress can be maintained and also no problems of break or warpage may be caused. Such control, however, is difficult since a very delicate control is required. Also, in practice, complicated steps must be taken such that the heating is carried out multi-stepwise from lower temperatures to high temperatures. Hence, this process can not produce SOI substrates which are feasible for mass production. To solve such problems, it is desired to obtain SOI substrates by polishing (or grinding) without the step of heat treatment. In other processes also, there are no techniques that can provide, in a good productivity, SOI substrates which are satisfactory for the fabrication of high-performance electronic devices.
As an additional problem, when single-crystal semiconductor substrates are produced by conventional deposited film forming processes, not a few stacking faults may occur in single-crystal deposited films. In such a case, the layer defects become larger as the deposited films grow. This is diagrammatically illustrated in FIG. 10. Reference numeral 1000 denotes a semiconductor substrate made of silicon or the like; 1002, an epitaxial growth layer; and 1009, layer defects. On the surface of the substrate 1000, point defects, dust, oxide residues and so forth may remain, and they cause the stacking fault. As the single-crystal semiconductor layer 1002 is epitaxially grown, the stacking fault grows to an extent increasing toward the end in the epitaxial layer because of the point defects, dust, oxide residues and so forth. Hence, the layer deposition defects may greatly extend on the surface of the epitaxial layer 1002.